1. Field of the Invention
The present invention relates to a communication system, and more specifically, to a communication system including a plurality of audio/video devices, a central control unit and a common input terminal interconnected through communication lines.
2. Description of the Related Art
Prior art audio/video communication systems interconnect a plurality of audio/video devices (units) including one or more audio signal source (such as AM/FM tuners, cassette players and compact disk (CD) players) and/or one or more video signal source (such as television receivers and navigation systems), an input terminal (operation unit) provided as a user interface, and a central control unit, all of which being interconnected through communication (bus) lines. In such systems, switching from an active (playing) audio/video device to an inactive (selected) audio/video device, turning on/off of power to the respective devices and transmitting operation changes (such as volume control and channel seek), which are sent to the respective sources from the operation unit, are transmitted entirely through the central control unit.
FIG. 20 conceptually shows a prior art system, wherein numeral 1a denotes an operation unit, numeral 2 denotes a central control unit, and numerals 3a, 3b and 3c denote a plurality of audio/video devices (audio signal sources or video signal sources). In the prior art system, when a user wishes to activate an AM/FM tuner (which may be the source 3a), an AM/FM tuner key is depressed on the operation unit 1a. In accordance with the key operation, the operation unit 1a transmits a request signal the central control unit to activate the AM/FM tuner 3a. If other audio units are in operation when this request signal is received, the central control unit 2 transmits a first command signal to turn off the operating audio units and then transmits a second command signal to activate the AM/FM tuner 3a. Further, when, for example, a band switch key, a preset channel tuning key or a seek key are depressed at the operation unit 1a while the AM/FM tuner 3a is in operation, respective control request signals are sent to the central control unit 2. The central control unit 2 then transmits command signals to control the AM/FM tuner 3a based on these control request signals to switch a band, tune a preset channel and effect seeking operation. Such control signals are used to control all of the audio sources (such as a tape player or a CD player) and all of the video sources.
The above-mentioned control method, however, has a deficiency in that it cannot support additional new sources and new operation units. For example, in order to connect a navigation system, a CD changer and their corresponding operation units to the above-mentioned system, the central control unit 2 must be provided with circuitry which can support the operation signals required for executing the special operations associated with the navigation system and the CD changer. However, it is difficult to foresee the operation requirements of new types of audio/video devices such that the central control unit can communicate with the new devices. Eventually, as new devices are connected, the prior art system becomes unable to support the addition of new devices and their corresponding operation units.
Further, a system installed, for example, on an automotive vehicle includes multiple operation units which are located in both the front and rear seat areas of the vehicle, or can be operated through a remote control unit. This arrangement is indicated by a plurality of operation units 1a, 1b and 1c which are indicated as connected to the central control unit 2 by dotted lines in FIG. 20. In this case, since various request signals are issued to the central control unit 2 from the respective control units, the burden on the central control unit is increased. Thus, there arises a problem that processing cannot take place at high speed, and a long time is necessary to switch signal sources and to execute command operations, and sometimes operation request signals are ignored.
Further, when power supplied to the respective audio/video devices or the control unit is momentarily interrupted for any reason and then restored in the system described above, the following problems arise:
A first problem arises when the power supplied to a selected device being played is momentarily interrupted and then restored while the power supplied to the central control unit is uninterrupted. The selected device stops because the internal state thereof is initialized (reset) when the power supply is restored. Although it is the control unit that controls the selected device, the control unit's memory contains information that the selected device is being played (not stopped by the interruption of the power supply). In other words, since the control unit recognizes that the device is being played, the control unit does not transmit a "play" control signal to the selected device. Therefore, the device is unable to output sound when the power supply to the device is restored. In some systems, even if a "play" key on an associated control unit is operated to select the device, since the control unit recognizes that the device is being played, it does not transmit the control signal to the device to enter the play state. In this case, it is necessary to briefly select another device, and then select the original device by which operability is deteriorated.
A second problem arises in a case that the power supply to a control unit is momentarily interrupted and then restored while power supplied to a selected (playing) device is uninterrupted. Since the control unit initializes (resets) its internal state when the power supply is restored, the memory of the control unit does contain information regarding which device is being played. Thus, there is a problem that the control unit cannot stop the play state of the source, and the source continues to freely play.
Incidentally, the control unit is typically arranged such that when the selection of a new source is commanded by a control unit or the like, the control unit at first transmits a command signal causing the selected device being played to stop, and then the control unit transmits a command signal causing the selected device to play. Since the control unit cannot recognize which device is being played when the power supply is restored, there is a problem that the selected device cannot be caused to play or, if the control unit subsequently commands a different device to play, two sources are caused to play at the same time.
In view of the above-mentioned problems, in accordance with the present invention, a communication unit is capable of reducing the burden on a central control unit and further supporting the addition of new sources and new operation units.
Also in accordance with the present invention, a communication system is capable of automatically controlling a source to enter a play state when the power supply to the source being played is momentarily interrupted and then restored, and a communication unit is capable of automatically stopping a source being played and further causing the memory content of the control unit to coincide with the operating condition of the system when the power supply to the control unit is momentarily interrupted and then restored.